The present invention relates to semiconductor structures, and particularly, to antifuse structures having an integrated heating element and methods of programming thereof.
Electrically operable fuses are utilized within the field of integrated circuit devices and processes for a number of purposes, including programming alterable circuit connections, or replacing defective circuit elements with redundant circuit elements. One type of electrically operable fuse, a so-called “antifuse”, is a device having two conductors and an intervening dielectric layer, where the dielectric layer is subject to breakdown upon application of sufficient voltage and current to the conductors. The resistance across the dielectric layer of the antifuse encodes the “on” or “off” state of the antifuse.
A typical (pre-breakdown) “off” resistance for antifuses having a dielectric layer of silicon nitride (SiN), “gate oxide”, i.e. silicon dioxide (SiO2) formed by the gate oxide forming process, or silicon oxide-silicon oxynitride-silicon oxide (ONO) is more than 1 GΩ. After breakdown, resistance across the dielectric layer is measurably lower, indicating the “on” state. Thus, the on-off state of the antifuse is read using a resistance measuring circuit.
At present, a high voltage and a current of several milliamperes may be required to adequately break down the dielectric of antifuses on an integrated circuit. Such required high currents impose minimum size constraints on the antifuses and wiring thereto, thereby requiring significant integrated circuit area to implement, while also negatively affecting the flow of production testing and repair of new chips. Provisions must also be made to safeguard the integrated circuit from being negatively affected by the required high programming voltage. The high programming voltage may give rise to concerns for electrostatic discharge protection (ESD) and the reliability of the integrated circuit.
In order for the state of an antifuse to be reliably read, the post-breakdown resistance must be in the megaohm range or below and, for yield reasons, this must be achieved for virtually all of the antifuses on the integrated circuit. Gate oxide antifuses typically require currents in the several milliampere range to achieve such post-breakdown resistance. However, such currents and the required high voltage are close to integrated circuit design constraints based on ESD protection and reliability considerations.
Antifuse technology through the use of dielectric breakdown is well understood. For example, U.S. Pat. No. 5,250,459 (the '459 patent), issued to Lee and entitled “Electrically Programmable Low Resistive Antifuse Element” embodies this concept. FIG. 1 of the '459 patent illustrates a conventional antifuse element 14 comprising a first electrode 11, a dielectric layer 12 and second electrode 13, all fabricated on substrate 10. To program antifuse element 14, that is to change the antifuse element from a high impedance state to a low impedance state, the conventional practice is to damage dielectric layer 12 by applying an electric field across dielectric layer 12 at first electrode 11 and second electrode 13. The electric field, if strong enough, will cause the dielectric layer 12 to breakdown, thus forming a conductive filament between first electrode 11 and second electrode 13. To reliably damage the dielectric layer 12, application of high programming voltages and currents are typically required. Gate oxide antifuses typically require several volts and currents in the several milliampere range to achieve such post-breakdown resistance.
This presents a problem in that the voltage/current required to program the antifuse must pass through standard CMOS logic without damaging it. One conventional solution, for example as described in U.S. Pat. No. 6,750,530 (“the '530 patent”), assigned to the assignee hereof and entitled “Semiconductor Antifuse With Heating Element,” is to form a heating element adjacent to, but not part of or in contact with, the antifuse element. Such a solution provides indirect heating, however, no component of the antifuse itself is involved in the generation of the heat. There are several drawbacks to such a solution utilizing indirect heating. First, additional processing steps are required to place a heat generation source in proximity to the antifuse. A resistive heating element (depicted as element 305 in FIG. 6B of the '530 patent) must be placed in the proximity of the antifuse (depicted as element 300 in FIG. 6B of the '530 patent). This requires additional process steps, thus increasing complexity and potential for yield loss. Second, although sufficient heat may be generated, transferring the heat to the antifuse is inefficient because of the indirect nature of the heating that occurs. For example, as illustrated in FIG. 6B of the '530 patent, to raise the temperature of antifuse dielectric layer 330, heat energy must radiate from heating element 305 through thick dielectric layer 340, which is about 0.5 microns thick. This heat transfer path is inefficient and requires a high programming current to travel through heating element 305 to produce sufficient indirect heating of dielectric layer 330. Additionally, the heat energy will disperse radially from heating element 305, thus further reducing the amount of heat energy that will reach the dielectric layer 330. Also, some amount of delay will occur from a point in time when the external heating element is activated to when the heat energy reaches the antifuse element. This delay is a function of both the distance between the external heating element and the antifuse element and the heat transfer characteristics (e.g. thermal conductivity) of the dielectric material that separates the external heating element and the antifuse element. The dielectric material that separates the external heating element and the antifuse element is typically a poor thermal conductor. Heat loss will occur as the heat energy passes through the insulator. Therefore, the size of the external heating element will have to be increased to account for such heat loss. Finally, the overall size of the programmable circuit is increased by adding a separate heating element, thus negatively impacting the size of the integrated circuit on which such antifuse structures reside.
Therefore, a need exists for an integrated, self-heating, less complex, reduced size, and more efficient antifuse structure where the antifuse dielectric layer is heated directly by the antifuse structure itself, not by an external heating element.